A solar cell is capable of directly converting solar light energy, which is clean and unlimited in supply, to electrical energy, and thus has been expected as a new energy source.
In general, because an electrical output of one sheet of a solar cell is small, a predetermined practical electrical output is generated from a solar cell module in which plural sheets of solar cells are connected to each other in a serial and parallel manner. The serial and parallel connection is performed by connecting a joining member to a negative electrode and a positive electrode.
In a double-sided electrode type solar cell in the related art, a plurality of thin wire electrodes (finger electrodes) that collect carriers and a connecting electrode (a bus bar electrode), which connects wiring materials are provided on a photoelectric conversion unit, and electrodes having a different polarity, that is, a P-type electrode and an N-type electrode, are provided on a front surface that is a light-receiving surface and a back surface, respectively. To form the solar cell module using the double-sided electrode type solar cell, it is necessary for a plurality of solar cells to be arranged in parallel and to be connected in series. However, at this time, it is necessary to join the front surface electrode, and the back surface electrode of an adjacent solar cell with a joining member.
In recent years, according to an increasing demand for further higher efficiency, development of a solar cell called a back contact type has been actively performed. In this solar cell, since back surfaces may be connected to each other when connecting a plurality of back contact type solar cells, a connection member is not provided on a front surface, light-receiving efficiency is improved, and external appearance becomes satisfactory. The P-type electrode and the N-type electrode are formed on the back surface of the back contact type, but the P-type electrode and the N-type electrode are configured to alternately enter between combs different from each other.
In this manner, the P-type electrode and the N-type electrode are alternately and densely formed. Due to this, carrier loss decreases, and thus high power generation efficiency is realized. Therefore, a factor of realizing high efficiency is to increase an area, in which the P-type electrode and the N-type electrode are present together, as much as possible.
Next, problems related to the reliability of the solar cell will be described below.
As reliability of the solar cell, generally, 90% or more of an initial value is guaranteed as the maximum output after 10 years, and 80% or more thereof is guaranteed as the maximum output after 20 years. However, accompanying an increase in an introduction amount of solar power generation, a phenomenon, which is not preferable from a reliability aspect, has obviously been occurring.
As an example, occurrence of a defective phenomenon called “hot spot” may be exemplified. This phenomenon is a heat generation phenomenon that occurs due to a difference in a generated output distribution at a part of solar cells and a module in a solar power generation system, and may cause significant damage to the system. Furthermore, it has been reported that occurrence of cracking of the solar cells is confirmed, or due to abnormal heat generation in a part of the solar cells and a connection casing, a solar cell surface is burned, or deformation and firing of the connection casing occurs.
Although various causes may be exemplified, it is considered that a joint failure between the solar cell and the joining member becomes one cause of the defective phenomenon (see, Patent Literature (hereinafter, abbreviated as PTL) 1, for example). Under a long-term use environment, when cracking extends at a solder joint portion between the solar cell and the joining member, power collection efficiency decreases, and thus a difference in a generated power distribution of the solar cell and the module occurs. Therefore, the heat generation phenomenon such as the hot spot occurs.
This joint failure becomes further significant in a solar cell of a thin substrate. For example, since a warpage state of the solar cell, which is caused by thermal stress during soldering of the above-described joining member mainly having a thickness of approximately 300 to 500 μm and less than 200 μm, increases, a joint failure such as electrode peeling-off easily occurs, and thus the reliability of the joint portion is further decreased.
In addition, electrons and holes, which are generated at the solder joint portion of the above-described back contact type solar cell, are proportional to an area of the joint portion, and a percentage of the electrons and holes, which are recoupled and thus disappear, increases. Therefore, the solder joint portion becomes an invalid power generation portion. To improve power generation efficiency of the back contact type solar cell, it is necessary to make the solder joint portion, which is an invalid power generation portion, relatively small, and to make the solder joint portion small, it is necessary to improve the reliability of each solder joint portion. As means for improving the reliability of the solder joint portion, when the thickness of a solder joining layer is controlled to be equal to or larger than a certain thickness and to be uniform, the reliability of the joint portion may be improved (see, NPL 2, for example).
In addition, in the joining between the solar cell and the joining member, a shape of the solar cell joining member and a joining method, which are configured to secure relatively higher reliability, are disclosed.
There is disclosed a method in which a concavo-convex shape is applied to the surface of the joining member, the joining member is uniformly heated due to the concavo-convex shape, and thus the joint portion having high reliability is accomplished (see, PTL 1, for example). When the concavo-convex shape is applied to the surface of the connection member, heating is easy and thus the joining between the solar cell and the joining member may be accomplished by treatment in a short time.
The reason is because an effective area of a covering material is further enlarged compared to a surface that is not treated, and thus when being heated with halogen or hot air, necessary heat may be obtained in a relatively easy and fast manner. When the covering material is uniform, the entirety of a layer is uniformly heated, and thus the joining between the solar cell and the joining member may be realized in a wide range.
In addition, a method in which a through-hole is provided to the joining member at a central position of the solder joint portion, a solder layer, which is positioned under the through-hole and on the electrode, is directly heated through the through-hole by a heating apparatus, and thus the joining member and the electrode are solder-joined is also disclosed (see, PTL 2, for example). Since the solar cell and the joining member are locally heated instead of heating the entirety thereof, a warpage state of the solar cell due to heat stress during soldering is mitigated, and thus shear stress at the joint portion is also mitigated. As a result, the reliability of the joint portion is improved. However, to secure the reliability of the joint portion, it is important to control the thickness of the joint portion or to reduce generation of voids by discharging a flux from the joint portion. However, this is not sufficient with a shape of the joining member in the related art.